1. The Field of the Invention
The present invention relates to systems for rotating discs in disc data stores. In particular it relates to such systems employing brushless D.C. motors where it is desired to brake the rotation of the disc or discs.
2. The Prior Art
It is well known to provide an electric motor to rotate one or more discs in a disc data store. The one or more discs have informational data recorded thereon in a plurality of concentric data storage tracks. The tracks are accessed for the recording or recovery of data by the positioning at selectable radii on the disc of a data recording or recovering transducer to interact with selected tracks. The disc or discs is or are generally magnetic in which case the transducer is a magnetic read/write head.
An important class of disc drives involves the use of a magnetic head which flies a few microinches over a thin magnetic film on a rapidly rotating disc supported by a thin layer of air pulled round by the disc in proximity to its surface. Whereas in the past it was the practice to withdraw the head from the surface of the disc before allowing the disc to cease rotating, with the advent of low cost rotary actuators for positioning the head over the disc for use in dust-free enclosures it has become difficult to do so and in the popular Winchester technology the head is actually moved to a parking track where it is allowed to land on the disc when the disc is stopped.
When landing a head on the disc there is always a risk of damaging the disc surface thereby throwing up debris to catch beneath the head and cause data errors on some other track or indeed to adhere to the head and adversly affect its flying characteristics. Debris can also induce wear on other tracks. The risk of damaging the landing zone is minimized if the rotation of the disc is rapidly braked thereby minimizing the time and distance of abrasive contact between head and landing track after the disc speed has dropped far enough to allow the head to cease flying.
It is well known to employ an electric induction motor to rotate the disc. The induction motor tends to be very large for the particular job in order to provide sufficient starting torque and is not very efficient during running thereby causing the generation of unwanted heat requiring to be dissipated. The induction motor provides little motor braking when power is removed and in order to brake the disc in reasonable time it is necessary to provide a separately operable brake. The brake is required to be very large and is therefor undesirable.
As an improvement over the induction motor, it is also known to provide a D.C. commutator motor for rotating the disc. The commutator motors tend to be smaller and more efficient for the same service. When it is desired to stop the disc a large amount of motor braking is available capable of bringing down the speed of the disc to a low enough level in an acceptably short time for a small mechanical brake to bring the disc to a complete halt. Commutator motors however suffer from commutator arcing which induces noise voltages into the data channel of a disc drive so causing data errors and which can contravene R.F.I. standards.
It has therefore become the practise to employ brushless D.C. motors for rotating the disc wherein hall effect switches take the place of the commutator to switch power via power transistors and the like to the windings. The control of brushless D.C. Motors is relatively complicated. The necessity for disc braking imposes a yet further burden.